FIG. 1 is a circuit diagram showing a configuration of a conventional resonance-type multiple-output switching power supply unit. In the primary side of a transformer T1 included in this multiple-output switching power supply unit, a full-wave rectifying circuit 2 rectifies an alternating current voltage from a commercial power supply 1. A smoothing capacitor C3 is connected between the output terminals of the full-wave rectifying circuit 2, and thus smoothes an output from the full-wave rectifying circuit 2. A first switching element Q1 and a second switching element Q2 are connected together in series between the two terminals of the smoothing capacitor C3. Thus, as a direct current input voltage Vin, a voltage between the two terminals of the smoothing capacitor C3 is applied to the first and second switching elements Q1, Q2. A control circuit 10 controls the on and off of each of the first switching element Q1 and the second switching element Q2. A voltage resonance capacitor Crv is connected to the second switching element Q2 in parallel.
A series resonance circuit is connected to the two terminals of the voltage resonance capacitor Crv, and includes the primary winding P1 (its number of turns is denoted by reference sign N1) of the transformer T1, a reactor Lr1 and a current resonance capacitor Cri which are connected one after another in series. Note that the reactor Lr1 is made of a leakage inductance between the primary and secondary sides of the transformer T1, for instance.
In addition, the secondary side of the transformer T1 includes: a first rectifying/smoothing circuit connected to a first secondary winding S1 (its number of turns is denoted by reference sign N2) whose coil is wound in order to generate a voltage having a phase which is reverse to that of a voltage of the primary winding P1 of the transformer T1; and a second rectifying/smoothing circuit connected to a second secondary winding S2 (its number of turns is denoted by reference sign N3) whose coil is wound in order to generate a voltage having a phase which is reverse to that of a voltage of the primary winding P1 of the transformer T1.
The first rectifying/smoothing circuit includes a diode D1 and a smoothing capacitor C1. Thus, the first rectifying/smoothing circuit rectifies and smoothes a voltage which is induced by the first secondary winding S1 of the transformer T1, and outputs the resultant voltage, as a first output voltage Vo1, from a first output terminal. The second rectifying/smoothing circuit includes a diode D2 and a smoothing capacitor C2. Thus, the second rectifying/smoothing circuit rectifies and smoothes a voltage which is induced by the second secondary winding S2 of the transformer T1, and outputs the resultant voltage, as a second output voltage Vo2, from a second output terminal.
This multiple-output switching power supply unit further includes a feedback circuit 5 configured to feed back to the primary side a signal depending on the first output voltage Vo1. The input side of the feedback circuit 5 is connected to the first output terminal. This feedback circuit 5 compares a voltage between the two terminals of the smoothing capacitor C1 with a predetermined reference voltage, and thus feeds back an error voltage, as a voltage error signal, to a control circuit 10 of the primary side.
The control circuit 10 performs a PWM control by alternately turning on and off the first switching element Q1 and the second switching element Q2 on the basis of the voltage error signal fed back from the feedback circuit 5, and thereby controls the first output voltage Vo1 so that the first control output voltage Vo1 can be constant. In this case, as a control signal, a voltage for causing the first switching element Q1 and the second switching element Q2 to intermittently stop functioning for a dead time of approximately several hundreds of nanoseconds is applied to the gates respectively of the first switching element Q1 and the second switching element Q2. This makes the first switching element Q1 and the second switching element Q2 alternately turned on and off without making the on period of the first switching element Q1 and the on period of the second switching element Q2 happen at the same time.
Next, referring to a waveform diagram shown in FIG. 2, descriptions will be provided for how the conventional multiple-output switching power supply unit thus configured operates.
In FIG. 2, reference sign Vds(Q2) denotes a voltage between the drain and source of the second switching element Q2; Id(Q1), a current flowing through the drain of the first switching element Q1; Id(Q2), a current flowing through the drain of the second switching element Q2; I(Cri), a current flowing through the current resonance capacitor Cri; V(Cri), a voltage between the two terminals of the current resonance capacitor Cri; If(D1), a current flowing through the diode D1; and If(D2), a current flowing through the diode D2.
The first output voltage Vo1 is controlled, when the PWM control is performed on the first switching element Q1 by the control circuit 10 which receives the voltage error signal fed back to the primary side from the first rectifying/smoothing circuit through the feedback circuit 5. In this case, as described above, depending on the control signal from the control circuit 10, the first switching element Q1 and the second switching element Q2 alternately turn on and off while repeatedly stopping functioning for the dead time of approximately several hundreds of nanoseconds.
First of all, during the on period (for instance, between time t11 and time t12) of the first switching element Q1, energy is stored in the current resonance capacitor Cri through the magnetic inductance of the primary winding P1 of the transformer T1 and the reactor Lr1 (leakage inductance between the primary and secondary windings of the transformer T1).
Subsequently, during the on period (for instance, between time t12 and time t14) of the second switching element Q2, based on the energy stored in the current resonance capacitor Cri, a resonant current generated by the reactor Lr1 and the current resonance capacitor Cri flows, and the energy is thus transferred to the secondary side. In addition, the magnetic energy of the magnetic inductance of the primary winding P1 is reset.
More specifically, during the on period of the second switching element Q2, a voltage obtained by dividing the voltage V(Cri) between the two terminals of the current resonance capacitor Cri by the magnetic inductance of the primary winding P1 and the reactor Lr1 is applied to the primary winding P1. Once the voltage applied to the primary winding P1 becomes equal to (Vo1+Vf)×N1/N2, the voltage is clamped. Thereby, the resonant current generated by the current resonance capacitor Cri and the reactor Lr1 flows, and the energy is thus transferred to the secondary side. This makes the current If (D1) flow through the diode D1. While the voltage of the primary winding P1 is less than (Vo1+Vf)×N1/N2, no energy is transferred to the secondary side of the transformer T1. Accordingly, the resonance operation in the primary side only is performed by the magnetic inductance of the primary winding P1 of the transformer T1, the reactor Lri, and the current resonance capacitor Cri.
In general, the on period of the second switching element
Q2 is set at a time length which is determined by the on period of the first switching element Q1 with the frequency being fixed, or set at an arbitrary constant time length. When the on period of the first switching element Q1 is changed, the ratio of the duty of the first switching element Q1 and the second switching element Q2 is changed. Thereby, the voltage of the current resonance capacitor Cri is changed. For this reason, it is possible to control the amount of energy which is transferred to the secondary side.
Moreover, the first secondary winding S1 and the second secondary winding S2 are coupled together with the same polarity being shared between the two windings. During the on period of the second switching element Q2, while energy obtained from the first secondary winding S1 is being outputted as the first output voltage Vo1, energy obtained from the second secondary winding S2 is outputted as the second output voltage Vo2. This second output voltage Vo2 is almost equal to Vo1×N3/N2.
In reality, however, the voltage generated in the first secondary winding S1 is higher than the first output voltage Vo1 by a forward step-down voltage Vf of the diode D1, and the voltage generated in the second secondary winding S2 is higher than the second output voltage Vo2 by a forward step-down voltage Vf of the diode D2. For this reason, change in Vf due to the load fluctuation of each output deteriorates the cross regulation. On the other hand, in the case of a power supply unit whose specification makes its output voltages changeable, once one output voltage changes, the other output voltage also changes in proportion to it. This makes it impossible to output the multiple outputs directly from the windings.
FIG. 3 is a circuit diagram showing a configuration of another conventional multiple-output switching power supply unit. This multiple-output switching power supply unit includes a regulator 12 such as a dropper or a step-down chopper instead of the second rectifying/smoothing circuit shown in FIG. 1. By use of this regulator 12, this multiple-output switching power supply unit generates the second output voltage Vo2 from the first output voltage Vo1. Thereby, this multiple-output switching power supply unit stabilizes its outputs. This multiple-output switching power supply unit is capable of solving the problem of the cross regulation between the two outputs.
In a case where an output current corresponding to the second output voltage Vo2 is small, this multiple-output switching power supply unit allows its circuit to be constructed economically by use of the dropper. However, in a case where the output current corresponding to the second output voltage Vo2 is large, this multiple-output switching power supply unit requires its circuit to be constructed by use of the step-down chopper and the like. Additional installation of parts including a switching element, a choke coil and a control IC for this construction pushes up the costs, and increases the packaging area. Besides, because the switching elements turn on and off paths through which large currents flow, significant switching loss occurs, and noise inevitably occurs as well.
In the case of a switching power supply unit described in Patent Document 1, a rectifying diode, a switching element and a smoothing capacitor are connected in series to a second secondary winding of a transformer included in a current resonance-type DC-DC converter; based on a voltage of the smoothing capacitor, the switching element turns on and off; and a direct current output is thus outputted. In addition, a leakage inductance of the transformer, the rectifying diode, the switching element and the smoothing capacitor operate as a chopper circuit, and are thus capable of stabilizing the direct current output.
Patent Document 1: Japanese Patent Application Publication No. 2006-197755.